It is a demand within the field of structural composites to produce still larger composite structures. The size of wind turbine blades and spars for wind turbine blades is for example constantly being increased to an extent where preparation of one-piece members requires unacceptable resources. Such resources may for example be large processing times during laying of layers and large production facilities with regard to apparatus size and space requirements. It is therefore desirable to prepare the composite structures in smaller members and connect these members to form the final structure at a later stage and/or facility.
Members to be connected may be non-cured, partially cured such as pre-consolidate or fully cured, respectively. In general, the members are becoming increasingly rigid with the degree of curing.
Experimental work has shown that presence of voids in the interface between connected members is detrimental to the mechanical performance of the connection. As a part of the connecting procedure it is therefore typically attempted to remove gas from the interface. However, if at least one of the members is not rigid, i.e. not fully cured, a significant risk remains that gas may be entrapped between the members without any chance of removing it by for example applying a vacuum.
This is for example the situation in GB 2 378 995 A, where a connection between two members via a compressible composite material is disclosed. The compressible composite comprises a fibrous material and a resinous material. The bulk of the compressible composite is substantially devoid of air void. In use, the compressible composite member is placed between the members to be connected and then formed to resemble the shape of the gap between the members by forcing the resinous material to leave the compressible composite. As the composite structure between the members are substantially devoid of air voids and reinforced by fibres, it may typically possess a high mechanical strength, but the preparation method does not take into account the aforementioned substantial risk of entrapment of gas between the individual member and the composite structure. The weak region of the combined structure as described in GB 2 378 995 A is hence the interfaces between the compressible composite material and each of the members.
When the connection is furthermore bearing a load, such as most connections between members reinforced by unidirectional fibres in the longitudinal direction of the fibres, the sensitivity towards voids in the interface represents a major course of lack of process reliability.
There is therefore an urgent need for a method of connecting members without the risk of having voids in the interface. Furthermore, the connection between the members should be highly reproducible and reliable and possess good mechanical strength.